The AnswerMug Challenge Series, Fork For Life, Knife For Life, or Spoon For Life Edition:

A knife. It is difficult to cut various foods, such as meats, with a spoon or fork. It can also be used for cutting many other things.

A fork. It's the best option for eating politely in the company of others. Knives are useful for cutting, but you wouldn't put a sharp knife in your mouth. Both spoons and forks put limitations on which foods you can eat, but there are other ways to enjoy soup and ice cream without using a spoon whereas if you need to spear a bite of food, such as salad, a fork is the way to go.

A spoon because peas always roll off a fork when I try to feed myself and they won't stay on a knife either.

A spoon.  (Ever try eating broth with a fork?) 
As one ... uh..., shall we say, "puts more mileage on their odometer", the... um... the "usable format" of their nourishment intake doesn't remain as... uh... as "diverse".  One might want to take this into mind when choosing a solitary eating utensil. 

Spoon. I cannot think of anything I could not eat with a spoon.  

For me personally, it would depend on the situation. If it was a normal, everyday situation, I would choose a spoon. However, if it was a matter of survival, (gathering food, cutting down and cutting up items, etc) I would choose a knife. And because I chose a knife, fingers work well when eating what you prepared. :)

Spoon.  I didn't even think about this.  You can do all kinds of things with a spoon.  It's the only one with which you can eat soups and I do love soups.

Knife

Unless you have a serrated fork or spoon, it's difficult to cut food with those utensils.
You can also use a knife to stab food, so it can replace the fork.
Anything I would use a spoon for, I could use a small cup.